When you calibrate a map you are situating it in the global space, in other words, you are situating it on the earth’s surface. To do this you have to know the exact coordinates of at least 2 or 3 points of the map. The bitmap image will be tallied to the coordinate system so that the information contained in the map coincides with the software’s reference system.
All maps used by Land/Air must have a calibration system incorporated so that they can best adjust the GPS’ indications to the coordinates of the map in view.